Cosmic-Ray Energy Distribution and Species of Cosmic-Ray Nuclei

The "ultra high energy" refers to the energy range
from 1012 eV - 1016 eV. The
energy distribution of ultra-high-energy cosmic rays,
summed over all the nuclear species, has a break point
around 4 x1015 eV. This point is called the
"Knee", and suggests the maximum energy up to which
cosmic rays are accelerated within their
source. Meanwhile, the standard theoretical model to
produce ultra-high-energy cosmic rays considers the
acceleration of cosmic rays by a shockwave
accompanying a supernova remnant in the Galaxy. Based
on this model, the maximum acceleration energy of a
cosmic-ray nucleus is linearly proportional to its
atomic number (Z). We can therefore verify the
shockwave acceleration model of the supernova remnant,
if we measure the energy distribution of each
cosmic-ray nuclear species separately, and observe
their linearly Z-dependent Knee energies.

Supernova remnant "SN 1006" in Lupus, located at a
distance of ~7,200 light years from the Earth. The
progenitor supernova of this nebula appeared in the
year 1006.
Image Credit: NASA, ESA, Zolt Levay (STScI)